1. Field of the Invention
The present invention relates to a matching circuit chip, a filter with a matching circuit and a duplexer mainly used for high-frequency apparatuses such as cellular phones.
2. Description of the Related Art
Conventionally, a duplexer comprises a high-impedance transmission line 2004 connected between a receiving filter 2006 and an antenna terminal 2002, and a high-impedance transmission line 2005 connected between the antenna terminal 2002 and a transmitting filter 2007 as shown in FIG. 21. Each of the transmission lines 2004 and 2005 is used to reverse the phase of the pass band frequency of its mating filter, thereby to obtain a high impedance condition at high frequencies. The transmission line 2004 is set so that the impedance of the receiving filter 2006 becomes open at the pass band frequencies of the transmitting filter 2007, and the transmission line 2005 is set so that the impedance of the transmitting filter 2007 becomes open at the pass band frequencies of the receiving filter 2006. As a result, a signal to be transmitted from the transmitting terminal 2003 to the antenna terminal 2002 is not affected by the receiving filter 2006, and a signal to be transmitted from the antenna terminal 2002 to the receiving terminal 2001 is not affected by the transmitting filter 2007. The circuit is thus used as a duplexer operating at a desired band.
In this kind of conventional duplexer, lines are required to be formed within a substrate having a low dielectric constant so that the transmission lines thereof have a sufficiently high impedance, thereby causing a problem of making the lengths of the lines longer and making the size of the duplexer larger. In addition, in the case when chip components are used instead of the transmission lines to form a matching circuit, problems are also caused; the number of components increases, and a frequency band wherein impedance matching can be attained becomes narrow.